1. Field of the Invention
The present invention relates to a hole inspection apparatus inspecting a plurality of holes formed in an inspected object such as a printed board or a substrate for a printed board.
2. Description of the Background Art
A printed board is formed with a large number of through holes (or via holes) passing through the printed board, in order to solder elements onto the printed board or the like. These through holes are generally formed by perforating prescribed positions of the printed board with a drill bit of a drilling machine. Such through holes must be reliably perforated, and hence inspection is performed as to presence/absence of a defect in any through hole.
The defect of the through hole is inspected as follows, for example: First, one surface of a substrate for the printed board is irradiated with illumination light for picking up an image of the rear surface thereof, thereby acquiring image data based on the light transmitted through the through holes. Information (hereinafter also referred to as “hole information”) as to the centroidal position of the through holes, the hole diameter, the hole area etc. is calculated from the image data for comparing the results of calculation with design data and detecting whether or not any through hole is defective. Correct inspection results can be obtained by extracting feature quantities in the picked-up image for executing such defect inspection.
While such defect inspection is preferably directed to all through holes, an extremely long time (e.g., two hours for one sheet) is disadvantageously required for calculating hole information as to all through holes over the entire image. Therefore, it is impractical to perform such defect inspection on all through holes, and the so-called “sampling inspection” extracting a partial area including partial through holes and inspecting only this partial area is performed in practice.
Following recent refinement of a wiring pattern formed on the printed board, however, the range of allowable errors for the through holes is reduced. More specifically, the width of land portions formed around the through holes is so reduced that the range of allowable positional errors is also reduced. The diameter of the through holes themselves is reduced, leading to reduction of the diameter of the drill bit itself. When a plurality of substrates are simultaneously perforated with such a thin drill bit, the drill bit is disadvantageously bent on an intermediate position, leading to a high possibility for a defect such as misregistration of the through holes in substrates located on lower portions.
The through holes are defected in a high possibility due to the aforementioned circumstances, and a probability of overlooking such defects is disadvantageously increased in the conventional sampling inspection. Therefore, total inspection for inspecting defects as to all through holes is increasingly required. In order to perform total inspection in practice, the time required for the inspection is desirably reduced by effectively performing the inspection. Such circumstances apply not only to the aforementioned case of providing “holes as through holes” but also to a case of forming general “holes” also including “holes formed in a state indented to intermediate positions along the thickness of an inspected object”.